Add platform billing: tiers, dues, payments and invoices

RosterChief charging the clubs, which is a different domain from `shop` (a club
charging its members). Nothing here is club-scoped: these rows reference a Club,
they are not owned by one, and no club user ever sees them.

- Tier + TierPrice. Prices are dated, not keyed by year: a rate change is one row
  with a future active_from, and price_on(day) answers "what was in force then".
  A tier with no price yet returns None, which callers must treat as "cannot
  bill" -- never as free.
- Due: one rolling-year period per club, with a 45-day grace tail. The tier and
  the amount are SNAPSHOTS taken when the period opens. Raise the price and last
  year's period must still say what was actually charged; reading it back through
  the tier would silently rewrite financial history.
- DuePayment: partial payments accumulate. amount_paid is re-summed from the
  payments on every change, never incremented -- an increment drifts the moment a
  payment is deleted, and the drift still looks like money.
- Invoice: PDF via WeasyPrint, rendered on demand from the frozen snapshot. Only
  the number is stored, in one platform-wide series (unlike the shop's per-club
  order numbers), and re-issuing returns the existing one rather than burning a
  number -- a gap in an invoice series is a question you don't want to answer.
  WeasyPrint is imported lazily: it binds to native pango/cairo, and the app, the
  tests and every other page must still run on a machine without them.
- archive_overdue_clubs reports by default and archives only with --commit. That
  asymmetry is deliberate: this switches off paying customers, so a bad clock or a
  cron misconfiguration should cost an email, not a morning of angry clubs. A club
  with auto_archive off is spared entirely.

Renewal continues from the last period end, not from the payment date: a club that
pays two months late has still used those two months.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pyproject.toml

@@ -12,6 +12,7 @@ dependencies = [
     "pillow>=12.3.0",
     "python-dateutil>=2.9.0.post0",
     "python-decouple>=3.8",
+    "weasyprint>=69.0",
 ]
 
 [dependency-groups]
@@ -49,7 +50,8 @@ ignore = [
 "rosterchief/settings/*" = ["F403", "F405", "E501"]
 
 [tool.ruff.lint.isort]
-known-first-party = ["authentication", "club", "members", "teams", "events", "formbuilder", "shop", "controlpanel", "news", "pages", "home", "search", "rosterchief"]
+known-first-party = [
+    "billing", "authentication", "club", "members", "teams", "events", "formbuilder", "shop", "controlpanel", "news", "pages", "home", "search", "rosterchief"]
 
 [tool.uv.sources]
 django-lucide = { git = "https://github.com/bsiebens/lucide" }